more general workaround for failed sessions (again, see also 2edb89630a80, ed7b5cd3a7f2);
\usepackage{ifthen}
\newcommand{\indexdef}[3]%
{\ifthenelse{\equal{}{#1}}{\index{#3 (#2)|bold}}{\index{#3 (#1\ #2)|bold}}}
\newcommand{\indexref}[3]{\ifthenelse{\equal{}{#1}}{\index{#3 (#2)}}{\index{#3 (#1\ #2)}}}
\newcommand{\isadigitreset}{\def\isadigit##1{##1}}
\newcommand{\isasystem}[1]{{\def\isacharminus{-}\def\isacharunderscore{\_}\isadigitreset\tt #1}}
\newcommand{\isatool}[1]{{\def\isacharminus{-}\def\isacharunderscore{\_}\isadigitreset\tt isabelle #1}}
\newcommand{\indexoutertoken}[1]{\indexdef{}{syntax}{#1}}
\newcommand{\indexouternonterm}[1]{\indexdef{}{syntax}{#1}}
\newcommand{\indexisarelem}[1]{\indexdef{}{element}{#1}}
\newcommand{\isasymIF}{\isakeyword{if}}
\newcommand{\isasymFOR}{\isakeyword{for}}
\newcommand{\isasymAND}{\isakeyword{and}}
\newcommand{\isasymIS}{\isakeyword{is}}
\newcommand{\isasymWHERE}{\isakeyword{where}}
\newcommand{\isasymBEGIN}{\isakeyword{begin}}
\newcommand{\isasymIMPORTS}{\isakeyword{imports}}
\newcommand{\isasymIN}{\isakeyword{in}}
\newcommand{\isasymFIXES}{\isakeyword{fixes}}
\newcommand{\isasymASSUMES}{\isakeyword{assumes}}
\newcommand{\isasymSHOWS}{\isakeyword{shows}}
\newcommand{\isasymOBTAINS}{\isakeyword{obtains}}
\newcommand{\isasymASSM}{\isacommand{assm}}